The present invention relates to high vacuum turbomolecular pumps and, more particularly, to a turbomolecular pump drive method and apparatus which reduces the temperature of operation of such a pump.
Turbomolecular pumps are increasingly being used to pump in the free molecule pressure range, i.e., 10.sup.-3 to 10.sup.-10 torr, because they are inherently non-contaminating. They operate on kinetic gas principles and rely on the relative motion between gas molecules and an interleaved cascade of stator and rotor blade wheels upon which such molecules impact. The design of the blade wheels makes it more probable that a molecule striking the same will rebound toward the pump outlet then toward its inlet.
In general, the pumping speed and efficiency of a turbomolecular pump is dependent upon the speed of rotation of the rotor blades. For this reason, rotor speeds as high as 50,000 r.p.m. are not unusual. The motor responsible for such high rotational speeds typically is incorporated directly into the pump with the motor rotor output shaft also serving as the axle or, in other words, spindle for the pump rotor.
The pump motor must rotate the large inertia load represented by the pump rotor at the reduced pressures within the pump casing. This results in significant heat build-up in the pump due to electrical inefficiencies in the motor. Such generation of heat can be so severe to limit the operating capabilities of turbomolecular pumps.
The electrically caused, excessive temperatures generated in such a pump, can be eliminated easily in the stationary components of such a motor, by thermal conduction to a heat sink. However, the rotating components can be difficult to cool due to poor heat conduction paths. Because of the vacuum environment, heat removal by convection is practically non-existent and radiation heat transfer can at most amount to just a few percent of the total heat generated.
Synchronous motor drives are chosen by some turbomolecular pump manufacturers in view of the low heat generation which is inherent in a synchronous motor when it operates synchronously. Synchronous motors, however, have other drawbacks. They have comparatively low efficiency, particularly when operated at power loadings less than the full load rating, and centrifugal force loading of the permanent magnetic materials used in synchronous motor rotors limit the same to relatively low rotational speeds. Moreover, occasional malfunction of the controls of such motors can cause the same to operate out of synchronous speeds. Such an asynchronous operation generates large amounts of heat in the motor rotor with resulting damage or destruction.
In view of the above drawbacks, many turbomolecular pump manufacturers have turned to induction-type motor arrangements for their pumps. The stator of such a motor generates a rotating magnetic field at a frequency of rotation proportional to the frequency of alternating current which is applied thereto by a power source. The rotor of such a motor is positioned within the rotating magnetic field to rotate in response thereto. Typically the motor rotor rotates with a frequency of rotation which is less than the frequency of rotation of the rotating magnetic field generated by the stator. The motor output shaft torque is a function of this difference, which difference is called the slip frequency. The amount of heat generated in the rotor of such an induction motor is approximately proportional to the torque output multiplied by this slip frequency.
The stator of an induction motor is maintained stationary, and the heat which is generated by electrical inefficiencies thereat can be easily removed from the pump. It is more difficult to remove the heat generated at the motor rotor. One method of attempting to do so is to utilize the flow of lubricant to the rotor shaft bearings, not only to lubricate such bearings but also to cool the motor rotor. U.S. Pat. No. 3,877,546 describes such a cooling arrangement. There are several significant problems with the same, however. For one, the flow of coolant is dependent on a minimum rotor rotation since centrifugal action is relied on for pumping. In typical operation, large amounts of heat are generated at the rotor during pump start up, i.e., before the motor has reached a sufficient rotational speed to cause centrifugal pumping of the lubricant/coolant. Moreover, there is generally a conduction joint between the motor rotor and shaft or other structure within which the lubricant flows. Such a joint also reduces the efficiency of a lubricant cooling arrangement by being an imperfection in the heat conduction path. This is especially true since the motor is operating at a reduced pressure so that there is no gas or other air to aid heat transfer across imperfections in the joint.